1. Field of the Invention
The invention relates in general to a magnetic core for electrical inductive apparatus, and in particular to a non-conductive core band for such apparatus.
2. Description of the Prior Art
Electrical inductive apparatus, such as transformers and electrical reactors, include a magnetic core consisting of a plurality of electrical steel plates, called punchings, that are stacked in groups over a frame work and are bound together by some means. The usual means employed is a plurality of core bolt studs. The core bolt studs have the dual purpose of acting as guide pins during core assembly, and clamping the core punchings together after assembly. It follows that assembly of the transformer or electrical apparatus must include the additional step of punching the bolt holes and insulating the bolts to prevent short circuits in the magnetic flux path.
Besides the increased labor and materials necessary for bolting the core punchings together, there are numerous other disadvantages, including:
(1) When the punchings are placed over the bolts during assembly, they may not fall flat, thus tending to bind by friction and hang up on the bolt sides and the bolt threads. This sets up punching waves in the core at the bolt stud location. Clamping this wavey structure after the assembly is completed stresses the stress sensitive magnetic material, increasing core losses.
(2) The punchings have the bolt holes pierced through them prior to assembly. When the die employed in the piercing operation becomes slightly worn, a slight bolt hole edge burr may be formed. During assembly, this burr may scratch through the insulating layer on neighboring punchings and cause a short circuit within the core.
(3) Metallic core bolts must be insulated from the punchings and from the end frame. Any subsequent breakdown of core bolt insulation during the lifetime of the induction apparatus may cause a short circuit in the magnetic flux path, generating heat, high core loss and noise, and eventually transformer failure.
(4) Bolt holes in the punchings cause flux crowding which increases the core loss value for a given KVA rating and prevents the efficient use of superior grades of grain oriented steel for the core medium. Thus, improved electrical steel cannot be efficiently used to build an improved core due to the high destruction factor of the bolted core structure. Destruction factor being a measurement of the flux carrying capacity of the raw material used in the core minus the flux carrying capacity of the finished product used in the core after assembly.
(5) Bolting restricts oil circulation by crimping the oil ducts near the bolts.
Bolting of magnetic core punchings requires increased labor and materials. It may also impair the physical and magnetic qualities of the core, and promote uneven punching stack space factor and joint closure. Bolting of the magnetic core punchings may also increase the operating noise level of a transformer.
A metal band would seem an ideal replacement for the bolting process as it has great tension capability and resistance to abrasive wear. However, closed metal loops in the vicinity of a magnetic core present a short circuited winding to the induction device.
Prior art banding arrangements for eliminating the undesirable consequences of bolting the core punchings without the use of a solid metal band have included the use of a metal band with an insulating link, and the use of a non-metallic band with a metallic fastener. Both arrangements require the assembly of a band consisting of different materials and coupling elements. Both bands require a larger over-all band height due to the thickness of the metal fastener of the insulating link. Both bands contain metal, and any metal located in the proximate vicinity of a flux circuit is potentially detrimental as it is subject to electrical stress concentrations and corona discharges.
Accordingly, it would be desirable to provide a completely non-metallic core band including non-metallic fastening means approximating the strength, tension and abrasive wear resistance of a steel band.